1. Field of the Invention
This invention relates generally to the subterranean heating of oil-bearing earth formations, and pertains more particularly to a system and method for heating such formations with microwave energy.
2. Description of the Prior Art
The need for tapping more difficult underground reservoirs containing petroleum impregnated media, such as oil shales, tar sands and the like, has been recognized for a number of years. Although numerous procedures have been tried, many obstacles have interfered with the effective and efficient production of oil from the sands and shale, as well as other formations, that contain the heavy crude oil.
Perhaps the most notable of the prior art techniques has been the use of steam that is injected into the surrounding earth strata. To do this, costly steam generators are required and the steam generators consume appreciable quantities of fuel, which in turn increases the overall cost. Besides, extensive lengths of pipes through which the steam flows must be insulated, and even then considerable thermal losses are experienced. Of course, some oil fields lend themselves more readily to steam injection than do others. In this regard, some are too shallow. Others are too deep. Besides, the formation itself poses a problem in many instances because it must be sufficiently porous so that the steam can penetrate adequately; even then, a considerable amount of heat is lost and lowers the recovery rate of the fluid so that the claiming of the residual crude oil becomes increasingly more difficult to recover.
There is also the so-called fire flood method that has been employed for secondarily recovering crude oil products. This has been tried more on an experimental basis than commercially. This method is not only dangerous but also consumes a portion of the recoverable crude oil during the removal process. In this regard, close control must be exercised with respect to the in situ fires, these fires sometimes being deep beneath the earth's surface. Also, the well is subjected to "coking" or gumming of the formation due to the excessive temperatures that are used. As with the steam injection method, the fire flood method suffers the common shortcoming of leaving a considerable quantity of crude in the formation with the concomitant impediment to future recovery thereof that has been experienced to a large degree with the steam technique.
Electrically heating the heavy crude oil formations has also been attempted over a number of years and has attracted a number of adherents who have attested to the success of the method. However, when attempting to transfer heat exclusively by conduction, it naturally develops that some subterranean formations are poorer electrical conductors than other formations; by the same token, some formations are thermally inferior as well. Implementation of complex heating arrangements which have included multiple injection and recovery bore holes have met, along with other drawbacks, serious economic limitations. Consequently, none of the electrical heating systems have performed satisfactorily enough to warrant any wide-scale commercial utilization thereof.
As early as the 1950's, the concept of stimulating oil flow by reducing the viscosity of heavy crude oils with high frequency electromagnetic energy was envisioned. However, during the era, the application of microwave energy was still in its infancy. Many early efforts involving the use of microwave energy proved to be impractical because of technological limitations. Even more recently with the advent of improved microwave equipment, there has remained the problem of dielectric properties being exhibited by virtue of diverse geographical formations and the interaction of the high frequency energy therewith.
One salient explanation for the lack of growth, as far as recovering oil with high frequency radiation, lies in the fact that the oil recovery devices and systems have not taken into adequate consideration the various parameters of the petroleum industry. Thus, a number of prior art microwave systems have failed to address, expecting instead that the oil industry would adjust to the electronic innovations, the actual needs of the industry itself. This lack of cooperation between the manufacturers of the microwave equipment and the petroleum industry has manifested itself in various ways. One such way has been concerned with the design of electronic heating systems that are too fragile to be handled by typical oil well crews, and are also too delicate to be incorporated in oil wells of the usual type and size. Furthermore, a number of the prior art systems have been unduly complex and not easily adapted to typical oil field practices, for they were not rugged enough. Still further, some required elaborate instrumentation making use of a plurality of bore holes, and an important criteria that was lacking from a number of such systems was the capability of the recovery equipment to fit into bore holes of current size so that the bore holes would not have to be restructured in order to accommodate the elaborate electronic paraphernalia. Being able to fit in an existing well bore is highly important where the well is partially depleted and which must be reopened with the expectation of an ample economic return to the investor; the economics of the situation simply does not permit a restructuring of the existing well bore.
It should be noted that a secondary oil recovery system in order to realize any commercial acceptance must operate with a minimum of technological complications. For instance, the location of power tubes and requisite electronic hardware in the well bore itself leaves too much control to chance. It should be readily apparent that if any failure occurs, the entire pump string has to be pulled which costs both time and money. Even though the advantages of electronic exploitations, particularly those employing high frequency generation, of heavy oil environs have received the accolades of respective specialists in both the petroleum and microwave industries, the fact remains that there is still a need for a practical system and method that will be both effective and efficient.